Moho Mojo

Serpentine at Dolar Point

Day 1

I woke up. Had breakfast, packed my last bits of clobber – bag with geo hammer, various chisels, hand lens, compass-clinometer, square protractor, surveyors note book, as well as other toys – cameras, astrotrac (incase the clouds clear), ukulele, clothes, fruity malt loaf. All the essentials. Set up various lights on timers so it looked like I was at home randomly wasting electricity. Gave the neighbours the spare key.

The sun was shining, what could possibly go wrong? Four and a half hours later, my car started to make a most distressing squealing noise which stopped for a while when I tapped the brakes and then started up again ten seconds later. Not good. I did the last 20 miles expecting a wheel to fall off.

Coverack was the last word in quaint. It also has zero mobile phone reception. Some people – even me some of the time – would call this a blessing, but right there and then, I needed to call the RAC. Lucky me – Hostels International put payphones in their establishments. Half an hour later the man from the RAC was jacking my car up. He couldn't see a problem with the brakes, or anything else that should make a squeal, so he took it for a spin. Three miles later, he pronounced that there is nothing wrong with it. It's like going to the bloody doctors. Well I didn't imagine the squeal!!

Moho under water

Ah well, I'm was here at least – The centre of the earth. Well not quite. Coverack has the unique distinction of having a sequence of rocks exposed that represent the transition from the earths mantle to the earths crust (known in geoscience as the Mohorovicic discontinuity or Moho for short), and as a geoscientist I was here to see it. A problem immediately became apparent. It was at that moment, under water, and the tide would not be all the way out for another 5 hours. Bugger! Scrambling over the rocks behind the the lifeboat station (now a restaurant) at Dolar Point enabled me to get up close and personal with the mantle, and despite the dire predictions of a salty old sea dog who was fixing his foc'sle or something at the slipway, the tide had turned and after an hour or so there was a widening strip of pebble strewn beach to examine, but as the light was failing, I thought it might be more productive to visit in daylight.

Day 2.

Serpentine Pebbles

Breakfast at the hostel with a nice family from Wales was followed by a short walk down to the beach. It rained briefly. At last luck was with me – the tide was out and the sun started shining. I started my traverse from the harbour north of Dolar Point. What is immediately apparent is that the simple layer cake sequence I had imagined Peridotite/serpentine – Troctolite-gabbro- basalt, would not be immediately apparent. Hundreds of years of sub-aerial exposure followed by exposure to sea water had ensured that the shore line rocks possessed a routine brown-black colour – that's all of them. This was of course not strictly true. On closer inspection of both the exposures themselves and the adjacent pebbles it was possible to track the differences. The mind blowing realization that I was walking across the Moho (!!!) was tempered by the obvious fact that hundreds, thousands, maybe millions of people had already done so at Coverack over the years – it is a small seaside town – there is a beach – the equation is simple enough... (Sloshing seawater) x (sand and pebbles) = (screaming children)/(dog walkers)... But enough ruining my Moho Mojo! I've crossed the Moho and I knew I was doing it – so there!

Bastite Serpentine with Chrysotile (white asbestos) veins

To walk at low tide from the picturesque little harbour to the northern end of the beach takes you from the depths were the mantle peridotite reacted with water and heat to form the more easily flowing serpentine; across the Moho, represented at Coverack by the unfamiliar mottled red cumulate rock troctolite, the boundary between the mantle and the crust; and into the course grained mafic lower crust represented by gabbro and basalt dykes. All of it helpfully squeezed up 10km from the earths depths and laid end on at Coverack beach for those in the know to walk over and wonder at. The reality is not as clear cut. The exact points were mantle ends – Moho begins – Moho ends – crust begins are blurred by erosion, cross cutting intrusions this way and that, and a couple of geological imposters (several huge lumps of gabbro and basalt that appear were no gabbro or basalt should be – the remains of a former coastal defensive structure).

Gabbro Veins

Basalt dyke

Moving swiftly on. A short car journey (in the car with nothing wrong with it!!), in a howling gale and hideous rain shower, took me to the basement, the rocks onto which this slab of ophiolite was thrust at the closure of the Rheatic ocean. This is a sequence of what were once volcanics. The heat and pressure created by the collision of two continent sized sections of crust re-assembled the minerals into forms that were stable in such extreme conditions. The result is seen at Lizard Point itself – Man o war gneiss. This rock has been through a lot, and one look at the painful contortions that can be seen in the rocks at Lizard is enough to tell a double story – First of all the heat and pressure that caused the minerals to reconstitute themselves into gneiss in the first place, and then a pressure so great that it can cause solid rock to bend! The tide was coming in rapidly, and the swell was big, and the shower had started again, so finding a piece of Man o war gneiss on the tiny swatch of beach below the old lifeboat station was exciting, but I got a little bit with a nice s-bend in it to illustrate the meaning of pressure.

Contorted Man o war gneiss at Lizard

During yet another shower, food with the nice family from Wales who seemed to have adopted me by this time was eaten in a cafe at the top of the path. This was followed by a trip to Mullion Cove were exists the geological frontier between the materials from the mantle and ocean floor, and the sediment into which they smashed. The resulting rock is hornblend schist, and the cliffs on the north side of the cove are made of it, whereas those on the south shore are serpentine. By now the tide was 95% in, which left a strip of harbour beach about yeah wide on which to find a sample of hornblend schist, and it was getting more exciting by the second as huge waves raced to meet me and the frequent showers (did I mention the frequent showers?) turned to hail stones. Another bracing British seaside day. After several advances into the fray, and a corresponding number of retreats, I had a worthwhile sample of the hornblend schist and the serpentine, oh and wet feet too. The storm was getting worse, and I thought that discretion was probably the better part of valour. An almost endless phalanx of rain heavy clouds marched in from the Atlantic like an invading army, I was soaked to the skin, and I thought my day was about over.

Serpentine from Mullion Cove

Day 3

I got up. It was raining heavily. I went home.